MarkdownTextBlock: Tables style improvements

[jcoc611-microsoft]

Before:

After:

Improvements:

• Margins
• Single Border element for outer lines which makes it possible to have corner radius
• Header background (default color is lightest accent color, configurable)

[ShankarBUS]

Hi @jcoc611-microsoft

I have ported MarkdownTextBlock to Uno Platform and made some changes to it for my personal project.

https://github.com/symptum/Symptum/blob/dev/src/Symptum.UI/Markdown/TextElements/TableElement.cs

The changes I made include:

• Removed of MyTableUIElement and used a plain Grid instead.
• Applying the table border to the cell's grid rather than using a custom panel i.e. MyTableUIElement.
• CornerRadius for each cell is calculated based on its index.
• Added support for Grid Tables as well.
• Rows can be colored alternatively.
• Used Style instead of Brush, Thickness properties in MarkdownTheme allowing theme-awareness and more customizability.

[Arlodotexe]

Tested locally, I can verify that the expected improvement appears on both Uwp and Wasdk 🎉